Pyrogenically produced titanium dioxide is produced by means of high-temperature hydrolysis of a volatile titanium compound, usually titanium tetrachloride, in an oxyhydrogen flame. This method is described in German Patent DE-PS 870,242 (1953). The resulting products primarily exhibit an anatase structure. They are hydrophilic, very pure and extremely fine. The primary particles (DIN 53 206) exhibit a spherical form and diameters of 10-100 nm in electronmicroscopical micrographs. A commercially available product exhibits an average primary particle size of 30 nm. Defined agglomerates (according to DIN 53 206) do not exist. The surface of the particles is smooth and free of pores. There is only an outer, readily accessible surface. The specific surface according to BET can be between 20 and 100 m.sup.2 /g, depending on production conditions. The afore-mentioned commercial product exhibits a specific surface of 50.+-.15 m.sup.2 /g.
Because of its high purity, high specific surface and the lack of pores, pyrogenically produced titanium dioxide is used as a titanium-dioxide component or as carrier material in catalytic systems (V. Rives-Arnau, G. Munuera, Appl. Surface Sci. 6 (1980) p. 122; N. K. Nag, T. Fransen, P. Mars, J. Cat. 68, p. 77 (1981); F. Solymosi, A. Erdohelyi, M. Kocsis, J. Chem. Soc. Faraday Trans pp. 1, 77, 1003 (1981); D. G. Mustard, C. H. Bartholomew, J. Cat. 67, p. 186 (1981); M. A. Vannice, R. L. Garten, J. Cat. 63, p. 255 (1980), M. A. Vannice, R. L. Garten, J. Cat. 66, p. 242 (1980).
However, only powdery catalytic systems are discussed in the literature cited above. If pyrogenically produced titanium dioxide is to be used on an industrial scale in catalytic systems, it would be advantageous to convert the powdery product into shaped bodies.
Since pyrogenically produced titanium dioxide has an especially fine particle size, its formation into shaped catalytic carriers presents problems.
Published German Patent Application DE-OS 31 32 674 discloses a method for the production of pressed parts of pyrogenically produced titanium dioxide in which silica sol is used as binder. Ethylene glycol, glycerol, erytherites, pentitols or hexitols are used as auxiliary pressing agents. This method has the disadvantage that an undesired "covering" occurs in the pressed parts during large-scale production, that is, an outer layer separates from the pressed parts.
Moreover, when this method is used, the shaped bodies contain considerable amounts of SiO.sub.2 in addition to TiO.sub.2, which arises from the binding agent. This influences the effectiveness of the catalyst.
Published German Patent Application DE-OS 32 17 751 discloses the pressing of pyrogenically produce titanium dioxide in which organic acids or acidic-reacting salts are used as intermediate binding agents.
This method has the disadvantage that the nature of the surface of the pyrogenically produced titanium dioxide is changed by the treatment with organic acids or acidic-reacting salts.
Published German Patent Application P 38 03 894 also teaches titanium-dioxide pressed parts.
The known methods are disadvantageous because a binding agent must be used which remains in the pressed part. This leads to a change of the catalytic action of the very pure, pyrogenically produced titanium dioxide.
Methods using precipitated titanium dioxide have the disadvantage that impurities are introduced by the precipitation process which influence the catalytic action.